GraphQL ADR: Initialize the 'filtering' section

Add the currently known items to that section, which then leads to the
following sections about actions and the query parameters. These are to
be filled through follow-up commits, scope wise.

Signed-off-by: Marco Miller <marco.miller@ericsson.com>

doc/adr/0002-GraphQL.md

@@ -42,7 +42,13 @@ Expected [TSP][tsp] features support.
 
 #### Server-side filtering
 
-We need to envision which REST endpoints would this be, to then fully check GraphQL feasibility.
+* Current TSP `/table/{outputId}/lines` is the main endpoint supporting [trace-server][inc] filtering.
+* Such filtering is optionally requested by clients through TSP [Query Parameters](#query-parameters).
+* A model tagging the matching table lines and columns (or cells) gets returned as a response.
+* Server may also deduce the [applicable actions](#executing-actions) from those tags.
+* This is captured by `trace-server`'s `TableLine` abstractions and implementations.
+* GraphQL's ability to support these (e.g., event) table search filters is therefore discussed below.
+* Filtering or searching an event table is based on the requested search expressions and direction.
 
 #### Executing actions